This invention relates to a system and method for automatically sensing, monitoring and adjusting the concentration of a material carried in a body of fluid. More particularly, the invention relates to a system and method for automatically sensing, monitoring and adjusting the toner concentration within liquid solution in an electrographic printing environment utilizing self calibration.
The electrographic recording process, for which the method of this invention is particularly applicable, includes the steps of forming an electrostatic latent image upon a recording medium and subsequently making the latent image visible. The recording medium, usually provided in web form, has a dielectric and a conductive surface and may be a coated paper, a polyester based transparent film, or other suitable material on which an electrostatic latent image is formed by means of a plurality of writing electrodes or styli physically positioned on one side thereof to electrically address the dielectric surface as the medium travels therepast through a recording station. Opposite the dielectric surface of the recording medium there is a conductive surface which in some cases is grounded. When the potential difference between the conductive surface and the recording elements is raised above a threshold level, on the order of several hundred volts, an electrostatic charge is deposited on the dielectric surface of the recording medium as the medium passes by the recording elements.
Subsequently the latent image is made visible during the development step by applying liquid or dry toner to the recording medium. The recording medium is contacted by a thin film of developer material out of which the toner particles are electrostatically attracted to the regions of electrostatic charge on the medium. These toner particles often are suspended in a liquid solution at a preferred concentration. As many images are developed, the particles suspended in the liquid become depleted causing the concentration of the particles in the liquid to be reduced. Therefore, as will become apparent, it is important to monitor the depletion of these particles as the concentration of the liquid changes and to compensate for such depletions as they occur.
Electrostatic plotters are available in a monochrome mode, including a single recording station and a single development station dispensing a single color toner, usually black. Also, electrostatic color plotters are available to produce full color plots by the sequential overlaying of a series of separate color images (yellow, cyan, magenta and black) to produce a full spectrum of colors.
There have been three basic approaches to color separation imaging. In the first, a series of images are formed sequentially each by means of a dedicated recording head and development station. In the second, a single recording head forms each color separation image on the recording medium which is then advanced past one of several development stations. Then, the recording medium is returned to the recording head for receiving the next color separation image and is advanced to the next development station. This process of advancing and returning the recording medium through the apparatus minimizes the number of recording heads and obviates the need for their critical alignment with respect to one another.
The third approach to color separation imaging is set forth in U.S. Pat. No. 4,799,452 to Day, U.S. Pat. No. 4,987,429 to Finley et al. and U.S. Pat. No. 4,796,051 to Monkelbann et al. Here a single recording head forms each color separation image on the recording medium, as in the second approach described above, however only one toner fountain is used for development whereby all toners pass through the same fountain which is purged between colors.
Each of the electrographic systems described above have to compensate for the changes in the concentration of the toner. In liquid toner electrostatic plotters, toner concentration is often measured optically. The liquid toner is pumped between two closely spaced, parallel, clear windows, forming a thin layer through which light is passed. Toner concentration is proportional to the amount of light registered at an optical sensor. A full description of such a system is described in U.S. Pat. No. 4,222,497 to Lloyd et al. which is assigned to a common assignee and hereby incorporated by reference. Various other systems using this approach are described in U.S. Pat. Nos.: 4,981,362; 4,660,152; 4,166,702; 4,119,989; 3,807,872; 3,712,203; 3,698,356; 3,677,222; and 3,354,802. Typically, color electrographic systems have four such windows, one for each color (e.g. black, cyan, magenta, yellow).
The accuracy of concentration measurement is highly sensitive to the thickness of the toner layer, i.e. the "window thickness", as well as variations in the optical properties of the toner being measured. This requires very tight tolerances on the window (e.g. 20.+-.1 mil) which, realistically, can only be met by sorting parts. Calibration of a plotter would be performed on the assembly line to compensate for the initial properties of the elements being used in that plotter. This type of calibration process does not take into account element properties that change or degrade over time. In addition, any improvements in toner formulation which affect optical properties cannot be easily implemented. Therefore, it would be advantageous to have a method by which a plotter can automatically compensate, or "self calibrate" for variations in window thickness, and toner optical properties. This would improve the accuracy of concentration measurement, decrease the cost of manufacturing windows, and allow improvements in toner formulation to be easily implemented. Such a calibration could be performed upon request.
Another problem with optical toner concentration measurement is window staining by the toner over long periods of time. This causes attenuation of the light, which can be misinterpreted as increased toner concentration, leading to measurement error. The same effect is caused by variations and aging in the optics and electronics. Systems of the past have taken a measurement of the window in the absence of toner to get a measurement of the staining of the window. However, a preferred method for automatic compensation for these effects can be accomplished by measuring the light attenuation with 0% concentration toner (i.e. clear ISOPAR.RTM. from Exxon Corp.) being circulated through the window. Such a system could use one window for measuring all toner values, including the value for the clear solution, and problems of residual toner on the window would be decreased due to the flushing effect of the clear solution.
One skilled in the art knows that the attenuation of the light is a logarithmic function of the toner concentration. One way to determine toner concentration is to measure the light attenuation, then perform a logarithmic calculation. Electrographic plotters are typically stand alone machines which use a microprocessor, or CPU (Central Processing Unit) for control, therefore, the logarithmic calculation required would be done by the microprocessor. The toner concentration measurement needs to take place during plot generation, or run-time mode, because that is when the toner is flowing through the measurement window.
Unfortunately, taking logarithms takes much longer than simple calculations, like addition. The CPU is very busy doing other functions at the same time as the concentration measurement, for instance; moving paper, handling plot data and several other tasks. It is so busy that only a few milliseconds at a time can be devoted to sensing concentration. One possible way to handle the problem is to add a coprocessor chip to do the logarithmic calculation, but the cost of this type of solution is prohibitive.
Therefore it would be advantageous to have a method for implementing toner concentration measurement which requires no logarithmic calculations during the run-time mode. Such a method could implement a table look-up scheme which can easily be done in the few milliseconds the CPU has available. One approach could be to put log table information into the look-up table, but this would require a huge amount of memory. However, a preferred approach would be to have a method which uses a self calibration routine that measures the actual window, optics, electronics and toner in the machine and creates very small look-up tables during a calibration time. This calibration could be performed upon request when the machine is idle and the tens of milliseconds of computational time required for logarithms is available. The calibration routine could generate small tables which can be accessed quickly during the run-time operation.
These small tables could be specific to the calibrated properties for a given plotter, and also contain the information needed to compensate for further window staining and electronics aging. As will be seen, concentration measurement during plot generation could be reduced to looking up values in two tables and adding them together. This look up scheme and corresponding addition would require less memory and fewer computational cycles than a system calculating the logarithmic values on the fly. Furthermore, it will be seen that such tables can be recreated at power-on by storing several key parameters, used during calculation of the values for the look-up tables, thereby saving nonvolatile memory space.